Mount Vesuvius erupted on August 24, AD 79. A vivid eyewitness report is preserved in two letters written by Pliny the Younger to the historian Tacitus, who had inquired about the death of the commander Pliny the Elder. This ancient scientist had rushed from Cape Misenum to help the stricken population and to get a close view of the volcanic phenomena, but he died at the spot.

“The cloud was rising up; we could not know from where it was coming out; we were still far away; later we came to know that it was the Vesuvius. Its shape was pine like. As from a huge trunk the cloud sprang high into the sky and was broadening as sprouting with long branches. At first it was rocketed by a strong wind blowing from down, then, left without the gas support, and won by its own weight, the cloud expanded in a large umbrella, shining white and appearing dirty at intervals, spotted with mud and stones, changing according to the ashes or soil brought up with it… Ashes started falling hotter and thicker on the ship in our approaching; then pieces of debris and stones darken by fire; a sudden shoal reduced the depth of the water and we could not disembark….The houses were tottering, shaked by frequent and violent earth tremors, almost uprooted from their basements, waving here and there…”

This is a passage of Pliny the Younger, full of details in a dramatic succession, when Pliny the Elder, a renowned naturalist of the Roman age, died, eager to see by his own eyes the unusual phenomenon. This passage can be considered the oldest document of the modern volcano logy.

Here is another detailed report of the course of the events, written by the scientists who work over this subject.

“That dreadful and fatal eruption lasted almost three days. It started at noon on 24 August and only on 26 the sky turned to the usual color. The disaster claimed more than thousand victims (the archeologists counted recently thousand and forty four bodies). Pompei was covered by seven meters of ashes and debris. The near by city Herculaneum was scratched out by a stream of mud between fifteen and twenty five meters thick, that kept it buried for seventeen centuries.

But what happened in that appalling disaster?

No one expected the eruption. The Vesuvius was sleeping for centuries and its slopes were cultivated. But few days earlier earth had started shaking. Most probably the inhabitants did not bother about those tremors that were quite frequent. They were caused by the magma coming from below that was finding its path towards the surface. The magma chamber should measure about 2 or 3 cubic km and was located at 3 to 5 km depth.

About 1 pm. 24 August the phase of maximum intensity of eruption started. This is just what Pliny the Young described, naming it “at pine shape”. It has been calculated that this giant cloud, containing gasses, pumice stones, and slag, reached 17 km in height. When this cloud was no more supported by the gashing gasses, collapsed on the slopes of the volcano and in few hours buried Pompei. Falling ashes and water generated also enormous landslides that buried Herculaneum and other centers of the area.

Then the volcano granted a truce of about ten hours. It was a treacherous truce. Many inhabitants had come back to search and save some of their belongings. The volcano started priming another terrible phase of the eruption.

The magma chamber, now partially emptied, was filled by ground water. The temperature being the very high, the water was transformed in steam that incredibly increased the internal pressure. The mountain swelled, rose; the sea floor rose. Just at the dawn of 25 August there was a second very violent explosion followed by a terrible earth quake. A new cloud was raised up. A huge quantity of steam, gasses and ashes poured down on the slopes of the Vesuvius, covering and destroying whatever it met. All the inhabitants still present and alive died stifled. A tick ash blanket covered the entire gulf and darkened the sky. Some other explosion followed, less and less violent and then calm slowly came back, reigning on death and destruction”.